Initiation of an antigen specific immune response classically requires activation of T cells via the TCR/CD3 complex and delivery of a second signal e.g. via CD28 of CD40L on T cells. It has been recognized that constant costimulation via CD3 and either CD28 or CD40L results in exponential expansion of T cells in vitro with massive production of TH1 cytokines and CC chemokines. The latter appears to mediate rapid internalization of CCR5, a major co-receptor for HIV and SIV which renders expanded CD4+ T cells refractory to HIV/SIV infection. The aim of this project is to evaluate the capacity of CD3/CD28 expanded T cells collected prior to SIV infection but post influenza/tetanus toxoid immunization to reconstitute immune functions impaired by SIV infection in vivo.